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/*
* Copyright (c) 2014, Yawning Angel <yawning at schwanenlied dot me>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* * Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
package obfs4
import (
csrand "crypto/rand"
"encoding/binary"
"fmt"
"hash"
"math/rand"
"github.com/dchest/siphash"
)
const distSeedLength = 16
// InvalidSeedLengthError is the error returned when the seed provided to the
// DRBG is an invalid length.
type InvalidSeedLengthError int
func (e InvalidSeedLengthError) Error() string {
return fmt.Sprintf("hashDrbg: Invalid seed length: %d", int(e))
}
// hashDrbg is a CSDRBG based off of SipHash-2-4 in OFB mode.
type hashDrbg struct {
sip hash.Hash64
ofb [siphash.Size]byte
}
// newHashDrbg makes a hashDrbg instance based off an optional seed. The seed
// is truncated to distSeedLength.
func newHashDrbg(seed []byte) *hashDrbg {
drbg := new(hashDrbg)
drbg.sip = siphash.New(seed)
return drbg
}
// Int63 returns a uniformly distributed random integer [0, 1 << 63).
func (drbg *hashDrbg) Int63() int64 {
// Use SipHash-2-4 in OFB mode to generate random numbers.
drbg.sip.Write(drbg.ofb[:])
copy(drbg.ofb[:], drbg.sip.Sum(nil))
ret := binary.BigEndian.Uint64(drbg.ofb[:])
ret &= (1<<63 - 1)
return int64(ret)
}
// Seed does nothing, call newHashDrbg if you want to reseed.
func (drbg *hashDrbg) Seed(seed int64) {
// No-op.
}
// wDist is a weighted distribution.
type wDist struct {
minValue int
maxValue int
values []int
buckets []float64
}
// newWDist creates a weighted distribution of values ranging from min to max
// based on a CSDRBG initialized with the optional 128 bit seed.
func newWDist(seed []byte, min, max int) (*wDist, error) {
w := new(wDist)
w.minValue = min
w.maxValue = max
if max <= min {
panic(fmt.Sprintf("wDist.Reset(): min >= max (%d, %d)", min, max))
}
err := w.reset(seed)
if err != nil {
return nil, err
}
return w, nil
}
// sample generates a random value according to the distribution.
func (w *wDist) sample() int {
retIdx := 0
totalProb := 0.0
prob := csRandInstance.Float64()
for i, bucketProb := range w.buckets {
totalProb += bucketProb
if prob <= totalProb {
retIdx = i
break
}
}
return w.minValue + w.values[retIdx]
}
// reset generates a new distribution with the same min/max based on a new seed.
func (w *wDist) reset(seed []byte) error {
if seed == nil {
seed = make([]byte, distSeedLength)
_, err := csrand.Read(seed)
if err != nil {
return err
}
}
if len(seed) != distSeedLength {
return InvalidSeedLengthError(len(seed))
}
// Initialize the deterministic random number generator.
drbg := newHashDrbg(seed)
dRng := rand.New(drbg)
nBuckets := (w.maxValue + 1) - w.minValue
w.values = dRng.Perm(nBuckets)
w.buckets = make([]float64, nBuckets)
var totalProb float64
for i, _ := range w.buckets {
prob := dRng.Float64() * (1.0 - totalProb)
w.buckets[i] = prob
totalProb += prob
}
w.buckets[len(w.buckets)-1] = 1.0
return nil
}
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